Innate immune recognition of microbial products plays a critical role in protection from infection by bacteria and viruses. It is now appreciated tat mitochondria participate in the response to innate immune recognition in various ways. Mitochondria have been described as hubs for innate immune signaling, acting as scaffolds for signaling complex assembly. Moreover, accumulating evidence suggests that cellular metabolism and energetics, which are largely mediated by mitochondria, play a critical role in activation and function of innate immune cells. This laboratory has identified a mitochondrial protein, ECSIT (Evolutionarily Conserved Signaling Intermediate in Toll pathways), which appears to have both bioenergetic and immune functions. In this proposal, the aim is to develop a genetic model to attain a better understanding of how ECSIT links TLR activation with downstream mitochondrial functions in immune cells. ECSIT was originally identified as a component of the NF-?B signaling pathway downstream of the Toll/IL-1 receptors. It has since been shown that ECSIT is required for assembly of complex I of the mitochondrial oxidative phosphorylation system (OXPHOS), as well as for efficient respiration. In addition, a recent paper from this laboratory demonstrated that ECSIT also controls the production of mitochondrial reactive oxygen species (mROS) following engagement of TLRs 1, 2 and 4, and that this mROS is essential for efficient killing of phagocytosed bacteria. To date, these findings appear to be the first to describe direct communication between TLRs and mitochondria. To further study the role of ECSIT in mediating TLR signaling to the mitochondria, a traditional ECSIT KO mouse was generated; however, ECSIT deletion resulted in embryonic lethality. To overcome the consequent difficulty in studying the physiological function of ECSIT, conditionally-targeted ECSIT knock-out (CKO) mice have been generated. In this proposal, the intent is to develop and characterize the ECSIT CKO mouse, and then utilize this model to determine how LPS signaling through ECSIT regulates mitochondrial function. Specifically, experiments are proposed to characterize the ECSIT CKO mice under steady-state conditions and in response to LPS exposure (R21), as well as to utilize the ECSIT CKO model to define the role of ECSIT in linking LPS/TLR signaling to downstream mitochondrial function(s) (R33).